From 1885 when Lord Rayleigh first described the acoustic waves that travel along the earth's surface due to earthquakes to present day use in scanning and communication systems, scientists have been interested in the action of acoustic waves in solid materials. It was demonstrated several years ago by workers at Bell Telephone Laboratories that ultrasonic waves with frequencies of a billion hertz and up are capable of travelling several centimeters through a solid medium. Best known, of course, for the use of acoustic waves in a solid medium is the piezoelectric crystal. Compression of such a crystal generates an electrostatic voltage across it and, conversely, application of an electric field may cause the crystal to expand or contract in certain directions.
In recent years, the acoustic wave technology expanded rapidly after the development of the interdigital transducer, an efficient type of transducer for converting an electrical signal into an acoustic surface wave and for reconverting the acoustic wave back into an electrical signal. When such an interdigital transducer is placed on a piezoelectric material such as quartz or lithium niobate (LiNbO.sub.3), and a rapidly changing electrical signal is applied to the transducer, the piezoelectric material will vibrate in unison with the electrical signal, generating a sound wave.
Combining the use of acoustic waves and optical light sources is known as acousto-optics. Acousto-optics is commonly used for achieving modulation and scanning of laser beams. This is normally done by using bulk acoustic waves which produce index of refraction variations within a material. These index of refraction variations interact with a laser beam causing diffraction of the beam. By turning the acoustic wave on and off, the amount of diffraction can be changed, achieving modulation of the diffracted light. By changing the frequency of the acoustic wave, the direction of diffraction can be changed, resulting in scanning of the output beam in angle.
The same type of deflection and modulation has been achieved, as shown in the art, by interaction of the light with acoustic surface waves. Use of surface waves rather than bulk waves is advantageous because attenuation can be lower (resulting in potentially higher resolution from a scanner), acoustic velocity is slower (resulting in smaller devices), and high frequency transducers are easier to fabricate. However, achieving interaction of light with waves is difficult. In order to achieve interaction of the light wave with the acoustic surface wave, two modes of interaction can be utilized. The most common technique, which is described in the literature, is to couple light into a thin film waveguide on the device surface, interact with the surface wave, and then couple out. For example, see an article by Yoshiro Ohmachi in the Journal of Applied Physics, Vo/44, No. 9, September 1973, pp 3928-3933 entitled, "Acousto-Optical Light Diffraction in Thin Films". An alternative mode of interaction is to couple light into a thicker crystal through a side adjacent to the active surface, allow it to totally internally reflect off the active surface at near grazing incidence, and leave the crystal through the other adjacent face. Such a device is described in a copending application entitled, "Light Modulator/Deflector Using Acoustic Surface Waves", by Robert Sprague and Dror Sarid.
According to the present invention, a technique is described for using acoustic surface waves to produce light deflectors. The interaction between the light and the acoustic waves is achieved in either of the modes described above. Along the path of the travelling acoustic wave in the same active device are placed one or more repeater acoustic wave sources to boost the decaying acoustic wave back to its original power level, achieving enhanced scan resolution as compared to a single transducer device.